Light emitting device and driving method thereof

ABSTRACT

The light emitting device has a limiter transistor which is connected to a monitoring element, and an inverter an output terminal of which is connected to a gate electrode of the limiter transistor and an input terminal of which is connected to one electrode of the limiter transistor and the monitoring element. In the case where the monitoring element is short-circuited, the limiter transistor can be turned off by the inverter to correct a defect of the monitoring element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/573,426, filed Feb. 8, 2007, now allowed, which is a 371 ofPCT/JP05/014968, filed Aug. 10, 2005, which claims the benefit of aforeign priority application filed in Japan as Serial No. 2004-236094 onAug. 13, 2004, all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a light emitting device which has aself-light emitting element, and a driving method thereof.

BACKGROUND ART

In recent years, a light emitting device including a light emittingelement typified by an EL (Electro Luminescence) element has beendeveloped, and it is expected to be widely used, by taking advantage ofhigh quality, wide viewing angle, thin size, lightweight, and the likebecause of a self-light emitting type.

Such light emitting elements may have degradation with time and aninitial defect. Proposed is a method that an anode surface of a lightemitting element is wiped using a PVA (polyvinyl alcohol) porous bodyand the like, so that planarization and the removal of dust are achievedto prevent the degradation with time and initial defect (referred toReference 1).

[Reference 1]

Japanese Patent Application Laid-Open No. 2002-318546

DISCLOSURE OF INVENTION

A primary object of the invention is to solve the aforementioneddegradation with time and initial defect of a light emitting element bya new method that is different from Reference 1 aforementioned.

In view of the problem, according to the invention, a voltage or acurrent which is supplied to a light emitting element is corrected byproviding a monitoring light emitting element in a portion of a lightemitting device and taking into consideration the change of themonitoring light emitting element.

Specifically, one mode of the invention is a light emitting device whichhas a plurality of monitoring light emitting elements, a monitor linewhich monitors a change of a potential of electrodes of the plurality ofmonitoring light emitting elements, and a means for interrupting acurrent which is supplied to a short-circuited monitoring light emittingelement through the monitor line in the case where any one of theplurality of monitoring light emitting elements is short-circuited.

Another mode of the invention is a light emitting device which has amonitoring light emitting element, a monitor controlling transistor oneelectrode of which is connected to the monitoring light emittingelement, and an inverter an output terminal of which is connected to agate electrode of the monitor controlling transistor and an inputterminal of which is connected to the one electrode of the monitorcontrolling transistor and the monitoring light emitting element.

Another mode of the invention is a driving method of a light emittingdevice which has a monitoring light emitting element and a monitorcontrolling transistor which is connected to the monitoring lightemitting element, includes the steps of turning off the monitorcontrolling transistor when the monitoring light emitting element isshort-circuited.

To achieve the aforementioned driving method, an inverter is a circuitwhich has a function to turn off a monitor controlling transistor in thecase where a monitoring light emitting element is short-circuited.Therefore, the invention is not limited to the inverter and othercircuits having the aforementioned function may be used.

A monitoring light emitting element is produced in the same steps as aplurality of light emitting elements which are provided in a pixelportion, therefore the monitoring light emitting element and theplurality of light emitting elements have the same or almost the samecharacteristics with respect to an environmental temperature at which alight emitting device is set (merely referred to as ambienttemperature), and a change with time (since degradation is caused inmany cases, referred to as degradation with time).

The invention can provide a light emitting device in which luminancevariations due to the change in ambient temperature and the degradationwith time are reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing a light emitting device of the invention.

FIG. 2 is a view showing an equivalent circuit of a pixel of theinvention.

FIG. 3 is a view showing a layout of a pixel of the invention.

FIG. 4 is a view showing a cross section of a pixel of the invention.

FIGS. 5A and 5B are views showing a monitoring circuit of the inventionand potential of each lines, respectively.

FIGS. 6A and 6B are views showing a monitoring circuit of the inventionand potential of each lines, respectively.

FIGS. 7A and 7B are views showing a monitoring circuit of the inventionand potential of each lines, respectively.

FIGS. 8A and 8B are views showing a timing chart of the invention.

FIG. 9 is a view showing an equivalent circuit of a pixel of theinvention.

FIGS. 10A to 10C are views each showing an equivalent circuit of a pixelof the invention

FIG. 11 is a view showing an equivalent circuit of a pixel of theinvention

FIG. 12 is a view showing a panel of the invention.

FIG. 13 is a view showing a timing chart of the invention.

FIGS. 14A and 14B are views each showing a timing chart of theinvention.

FIGS. 15A to 15F are views each showing an electronic device of theinvention.

FIG. 16 is a view showing a light emitting device of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Although the invention will be fully described by way of EmbodimentModes and Embodiments with reference to the accompanying drawings, it isto be understood that various changes and modifications will be apparentto those skilled in the art. Therefore, unless otherwise such changesand modifications depart from the scope of the invention, they should beconstrued as being included therein. Note that in all diagrams fordescribing embodiment modes, same portions or portions which have asimilar function are denoted by like numerals and will be explained inno more details.

Note that in this specification, a connection between each element meanselectrical connection. Therefore, elements may be connected to eachother through a semiconductor element, a switching element and the like.

Further, in this specification, a source electrode and a drain electrodeof a transistor are names for distinguishing electrodes other than agate electrode for the sake of convenience because of a transistorconfiguration. In the case where the conductivity of a transistor is notlimited, the source electrode and the drain electrode are changed inname in the invention depending on the conductivity of the transistor.Therefore, a source electrode or a drain electrode may be described asany of one electrode and the other electrode.

Embodiment Mode 1

This embodiment mode describes a panel configuration which has amonitoring light emitting element.

In FIG. 1, a pixel portion 40, a signal line driving circuit 43, a firstscanning line driving circuit 41, a second scanning line driving circuit42, and a monitoring circuit 64 are provided over an insulatingsubstrate 20.

A plurality of pixels 10 are provided in the pixel portion 40, and eachpixel includes a light emitting element 13 and a transistor (hereinafterreferred to as a driving transistor) 12 which is connected to the lightemitting element 13 and functions to control current supply. The lightemitting element 13 is connected to a power source 18. Note that aspecific configuration of the pixel 10 is exemplified in the followingembodiment modes.

The monitoring circuit 64 has a monitoring light emitting element 66, atransistor (hereinafter referred to as a monitor controlling transistor)111 which is connected to the monitoring light emitting element 66, andan inverter 112 an output terminal of which is connected to a gateelectrode of the monitor controlling transistor 111 and an inputterminal of which is connected to one electrode of the monitorcontrolling transistor 111 and the monitoring light emitting element 66.A constant current source 105 is connected to the monitor controllingtransistor 111 through a monitoring current line (hereinafter referredto as a monitor line) 113. The monitor controlling transistor 111functions to control a current supply from the monitor line 113 to eachof the plurality of monitoring light emitting elements 66. Since themonitor line 113 is connected to electrodes of the plurality ofmonitoring light emitting elements 66, it can function to monitor achange of the potential of the electrodes. Further, the constant currentsource 105 may function to supply a constant current to the monitor line113.

The monitoring light emitting element 66 and the light emitting element13 are produced in the same steps under the same conditions, and thushave the same configuration. Therefore, the monitoring light emittingelement and the light emitting element have the same or almost the samecharacteristics with respect to the change in ambient temperature andthe degradation with time. Such monitoring light emitting elements 66are connected to the power source 18. Herein, the power source connectedto the light emitting element 13 and the power source connected to themonitoring light emitting element 66 have the same potential, therefore,they are denoted by the same reference numeral: the power source 18.Note that, in this embodiment mode, the conductivity of the monitorcontrolling transistor 111 is described as the P-channel type, thoughthe invention is not limited thereto, and the N-channel type may beused, in which case a periphery circuit configuration is changed atdiscretion.

A position in which such a monitoring circuit 64 is provided is notlimited and may be provided between the signal line driving circuit 43and the pixel portion 40, or between the first scanning line drivingcircuit 41 or the second scanning line driving circuit 42 and the pixelportion 40.

A buffer amplifier circuit 110 is provided between the monitoringcircuit 64 and the pixel portion 40. The buffer amplifier circuit is acircuit having characteristics such that an input and an output are atthe same potential, input impedance is high, and output current capacityis high. Therefore, a circuit configuration can be determined atdiscretion as long as it has these characteristics.

In such a configuration, the buffer amplifier circuit 110 functions tochange voltage which is applied to the light emitting element 13 in thepixel portion 40 in accordance with a change of the potential of oneelectrode of the monitoring light emitting element 66.

In such a configuration, the constant current source 105 and the bufferamplifier circuit 110 may be provided over the same insulting substrate20 or another substrate.

In the aforementioned configuration, a constant current is supplied fromthe constant current source 105 to the monitoring light emitting element66. In this condition, when ambient temperature change or degradationwith time occurs, a resistance value of the monitoring light emittingelement 66 is changed. For example, when degradation with time occurs,the resistance value of the monitoring light emitting element 66increases. Then, since a current value which is supplied to themonitoring light emitting element 66 is constant, a potential differencebetween both terminals of the monitoring light emitting element 66 ischanged. Specifically, a potential difference between both electrodes ofthe monitoring light emitting element 66 is changed. At this time,because the potential of an electrode connected to the power source 18is fixed, the potential of an electrode connected to the constantcurrent source 105 is changed. The change of the potential of theelectrode is supplied to the buffer amplifier circuit 110 through themonitor line 113.

That is, the change of the potential of the electrode is inputted to aninput terminal of the buffer amplifier circuit 110. A potentialoutputted from an output terminal of the buffer amplifier circuit 110 issupplied to the light emitting element 13 through the driving transistor12. Specifically, an outputted potential is given as the potential ofone electrode of the light emitting element 13.

Thus, a change of the monitoring light emitting element 66 due to achange of ambient temperature and degradation with time is fed back tothe light emitting element 13. As a result, the light emitting element13 can emit light with a luminescence corresponding to the change ofambient temperature and the degradation with time. Therefore, a lightemitting device which can display images independently of a change ofambient temperature and degradation with time can be provided.

Further, because the plurality of monitoring light emitting elements 66are provided, the potential changes thereof can be averaged and suppliedto the light emitting element 13. In other words, in the invention,potential changes can be averaged by providing the plurality ofmonitoring light emitting elements 66, which is preferable.

A monitoring light emitting element in which a short-circuit and thelike occurs can be replaced by providing the plurality of monitoringlight emitting elements 66.

Furthermore, in the invention, the monitor controlling transistor 111and the inverter 112 which are connected to the monitoring lightemitting element 66 are provided taking into consideration malfunctionof the monitoring circuit 64 due to a defect (including an initialdefect and a defect with time) of the monitoring light emitting element66. For example, in the case where the constant current source 105 andthe monitor controlling transistor 111 are connected without othertransistors and the like interposed therebetween, an anode and a cathodeof one monitoring light emitting element 66 of the plurality ofmonitoring light emitting elements are short-circuited by a defect andthe like in production steps. Then, a current from the constant currentsource 105 is supplied a lot to the monitoring light emitting element 66which is short-circuited through the monitor line 113. Since theplurality of monitoring light emitting elements are connected inparallel to each other, when much current is supplied to the monitoringlight emitting element 66 which is short-circuited, a predeterminedconstant current is not supplied to the other monitoring light emittingelements. As a result, an appropriate potential change of the monitoringlight emitting element 66 cannot be supplied to the light emittingelement 13.

An anode potential and a cathode potential of the monitoring lightemitting element become the same by such a short-circuit of themonitoring light emitting element. For example, in production steps,dust and the like between an anode and a cathode may cause ashort-circuit. Further, the monitoring light emitting element may beshort-circuited in the case of a short-circuited between a scanning lineand an anode and the like as well as a short-circuit between an anodeand a cathode.

In view of the foregoing, according to the invention, the monitorcontrolling transistor 111 and the inverter 112 are provided. Themonitor controlling transistor 111 stops current supply to themonitoring light emitting element 66 which is short-circuited to preventmuch current from being supplied to the monitoring light emittingelement 66 due to the short-circuit and the like. That is to say, themonitor controlling transistor 111 disconnects the short-circuitedmonitoring light emitting element from the monitor line.

The inverter 112 functions to output a potential at which the monitorcontrolling transistor is turned off when any one of the plurality ofmonitoring light emitting elements is short-circuited. In addition, theinverter 112 functions to output a potential at which the monitorcontrolling transistor is turned on when none of the plurality ofmonitoring light emitting elements are short-circuited.

An operation of the monitoring circuit 64 is described in detail withreference to FIGS. 5A and 5B. As illustrated in FIG. 5A, the monitoringlight emitting element 66 has an anode electrode 66 a on a highpotential side and a cathode electrode 66 c on a low potential side. Theanode electrode 66 a is connected to an input terminal of the inverter112. The cathode electrode 66 c is connected to the power source 18,which becomes a fixed potential. Therefore, when the anode and thecathode of the monitoring light emitting element 66 are short-circuited,a potential of the anode electrode 66 a becomes close to a potential ofthe cathode electrode 66 c. As a result, a low potential which is closeto the potential of cathode electrode 66 c is supplied to the inverter112 so that a P-channel transistor 112 p included in the inverter 112 isturned on. Then, a potential (Va) of a high potential side is outputtedfrom the inverter 112, which becomes a gate potential of the monitorcontrolling transistor 111. That is, a potential inputted to a gate ofthe monitor controlling transistor 111 becomes Va, and the monitorcontrolling transistor 111 is turned off.

Note that a high side potential (High) VDD is set to be equal to orhigher than an anode potential. Further, a low side potential of theinverter 112, a potential of the power source 18, a low side potentialof the monitor line 113, and a low side potential applied to Va can bemade equal to each other. In general, a low side potential is ground,though the invention is not limited thereto, and the low side potentialmay be determined to have a predetermined potential difference with ahigh side potential. The predetermined potential difference can bedetermined by current characteristics, voltage characteristics, andluminescence characteristics of a light emitting material, or devicespecifications.

Herein, the order of supplying a constant current to the monitoringlight emitting element 66 is paid attention to. It is required to startsupplying a constant current to the monitor line 113 while the monitorcontrolling, transistor 111 is in an on-state. In this embodiment mode,a current starts flowing to the monitor line 113 while keeping Va at Lowas shown in FIG. 5B. Then, Va is to be a VDD after a potential of themonitor line 113 reaches a saturation state. As a result, even thoughthe monitor controlling transistor 111 is in an on-state, the monitorline 113 can be charged.

On the other hand, in the case where the monitoring light emittingelement 66 is not short-circuited, a potential of the anode electrode 66a is supplied to the inverter 112 so that an N-channel transistor 112 isturned on. Then, a potential of a low potential side is outputted fromthe inverter 112 so that the monitor controlling transistor 111 isturned on.

Thus, a current from the constant current source 105 can be preventedfrom flowing to the monitoring light emitting element 66 which isshort-circuited. Therefore, in the case where one of the plurality ofmonitoring light emitting elements is short-circuited, current supply tothe monitoring light emitting element which is short-circuited isblocked so that a potential change of the monitor line 113 can beminimized. As a result, a current can be supplied to the light emittingelement 13 in accordance with an appropriate potential change of themonitoring light emitting element 66.

Note that in this embodiment mode, the constant current source 105 maybe any circuit as long as it can supply a constant current, and forexample, a transistor can be used.

Further, in this embodiment mode, described is the case where themonitoring circuit 64 has the plurality of monitoring light emittingelements 66, the monitor controlling transistor 111, and the inverter112, though the invention is not limited thereto. For example, theinverter 112 may be any circuit as long as it functions to detect ashort-circuit of the monitoring light emitting element and to blockcurrent supply to the monitoring light emitting element which isshort-circuited through the monitor line 113. Specifically, the inverter112 may function to turn off the monitor controlling transistor in orderto block a current supplied to the monitoring light emitting elementwhich is short-circuited.

Further, this embodiment mode is preferable since the plurality ofmonitoring light emitting elements 66 are used and even when one of themhas a defect, a monitor operation can be implemented. Further, theplurality of monitoring light emitting elements can average a monitoroperation, which is preferable.

In this embodiment mode, the buffer amplifier circuit 110 is provided toprevent a potential change. Therefore, other circuits than the bufferamplifier circuit 110 may be used as long as they can prevent apotential change similarly to the buffer amplifier Circuit 110. That isto say, when a potential of one electrode of the monitoring lightemitting element 66 is transmitted to the light emitting element 13, anycircuit as well as the buffer amplifier circuit 110 may be providedbetween the monitoring light emitting element 66 and the light emittingelement 13 to prevent a potential change.

Embodiment Mode 2

This embodiment mode describes, differently from the aforementionedembodiment mode, a circuit configuration in which a monitor controllingtransistor is turned off when a monitoring light emitting element isshort-circuited, and an operation thereof.

The monitoring circuit 64 shown in FIG. 6A has a P-channel firsttransistor 80, an N-channel second transistor 81 which has a common gateelectrode to the first transistor 80 and is connected in parallel to thefirst transistor 80, and an N-channel third transistor 82 which isconnected in series to the second transistor. The monitoring lightemitting element 66 is connected to gate electrodes of the firsttransistor 80 and the second transistor 81. A gate electrode of themonitor controlling transistor 111 is connected to an electrode to whichboth of the first transistor 80 and the second transistor 81 areconnected. The other configuration is similar to the monitoring circuit64 shown in FIG. 5A.

Further, it is assumed that a potential of a high potential side of thefirst P-channel transistor 80 is Va, while a potential of the gateelectrode of the third N-channel transistor 82 is Vb. A potential of themonitor line 113, and potentials Va and Vb are operated as shown in FIG.6B.

First, the potential of the monitor line 113 reaches a saturation state,thereafter, the potential Va becomes High. In the case where themonitoring light emitting element 66 is short-circuited, an anodepotential of the monitoring light emitting element 66, that is, apotential at a point D decreases to almost the same as a cathodepotential of the monitoring light emitting element 66. Then, a lowpotential, that is Low is inputted to the gate electrodes of the firsttransistor 80 and the second transistor 81, so that the N-channel secondtransistor 81 is turned off and the P-channel first transistor 80 isturned on. Then, a high side potential which is one potential of thefirst transistor 80 is inputted to the gate electrode of the monitorcontrolling transistor 111 to turn off the monitor controllingtransistor 111. As a result, a current from the monitor line 113 is notsupplied to the monitoring light emitting element 66 which isshort-circuited.

At this time, in the case where a short-circuit level is low and ananode potential decreases slightly, which of the first transistor 80 andthe second transistor 81 is turned on or off is difficult to becontrolled. Then, the potential Vb is supplied to the gate electrode ofthe third transistor 82 as shown in FIG. 6A. That is to say, while thepotential Va is at High, the potential Vb is at Low as shown in FIG. 6B.Then, the N-channel third transistor 82 is turned off. As a result, ifan anode potential is a potential obtained by subtracting a thresholdvoltage of the first transistor 80 from VDD, the first transistor 80 canbe turned on and the monitor controlling transistor 111 can be turnedoff.

By controlling the potential Vb in the aforementioned manner, even whenan anode potential decreases slightly, the monitor controllingtransistor 111 can be turned off accurately.

Note that, in the case where the monitoring light emitting elementoperates normally, the monitor controlling transistor 111 is controlledto be turned on. That is to say, an anode potential becomes almost thesame as the high potential of the monitor line 113 so that the secondtransistor 81 is turned on. As a result, the low potential is applied tothe gate electrode of the monitor controlling transistor 111 so that themonitor controlling transistor is turned on.

Further, as shown in FIG. 7A, the monitor circuit 64 has a P-channelfirst transistor 83, a P-channel second transistor 84 which is connectedin series to the first transistor 83, an N-channel third transistor 85which has a common gate electrode to the second transistor 84, and anN-channel fourth transistor 86 which has a common gate electrode to thefirst transistor 83 and is connected in parallel to the first transistor83. The monitoring light emitting element 66 is connected to the gateelectrodes of the second transistor 84 and the third transistor 85. Thegate electrode of the monitor controlling transistor 111 is connected toan electrode to which the second transistor 84 and the third transistor85 are connected. Further, the gate electrode of the monitor controllingtransistor 111 is connected to one electrode of the fourth transistor86. The other circuit configuration is similar to the monitoring circuit64 shown in FIG. 5A.

First, the potential of the monitor line 113 reaches a saturation state,thereafter, a potential Ve becomes Low. In the case where the monitoringlight emitting element 66 is short-circuited, an anode potential of themonitoring light emitting element 66, that is, a potential at a point Ddecreases to almost the same as a cathode potential of the monitoringlight emitting element 66. Then, a low potential, that is Low isinputted to the gate electrodes of the second transistor 84 and thethird transistor 85, so that the N-channel third transistor 85 is turnedoff while the P-channel second transistor 84 is turned on. In the casewhere the potential Ve is Low, the first transistor 83 is turned onwhile the fourth transistor 86 is turned off. Then, a high sidepotential of the first transistor 83 is inputted to the gate electrodeof the monitor controlling transistor 111 through the second transistor84, and the monitor controlling transistor 111 is turned off. As aresult, a current from the monitor line 113 is not supplied to themonitoring light emitting element 66 which is short-circuited.

The potential Ve of the gate electrode is controlled in theabovementioned manner so that the monitor controlling transistor 111 canbe turned off accurately.

Embodiment Mode 3

In the invention, a reverse bias voltage can be applied to a lightemitting element and a monitoring light emitting element. In thisembodiment mode, described is the case where a reverse bias voltage isapplied.

On the assumption that a voltage applied when the light emitting element13 and the monitoring light emitting element 66 emit light is a forwardbias voltage, a reverse bias voltage is a voltage in which a high sidepotential and a low side potential of the forward bias voltage areinverted. Specifically, when a description is made by using themonitoring light emitting element 66, the potential of the monitor line113 becomes lower than the potential of the power source 18 so that thepotential of the anode electrode 66 a and the potential of the cathodeelectrode 66 c are inverted.

Specifically, the potential of the anode electrode 66 a (anodepotential: Va) is changed from High to Low as shown in FIG. 13, and thepotential of the cathode electrode 66 c (cathode potential: Vc) ischanged from Low to High. At this time, a potential (V₁₁₃) of themonitor line 113 is also changed from High to Low. The period duringwhich the anode potential and the cathode potential are inverted isreferred to as a reverse bias voltage application period. After apredetermined reverse bias voltage application period passes, thecathode potential is returned from High to Low so that a constantcurrent flows to the monitor line 113 and charge is completed.Thereafter, that is after a voltage of the monitor line 113 becomesHigh, an anode line potential is returned from Low to High. At thistime, the potential of the monitor line 113 is returned in a curve withtime because the plurality of monitoring light emitting elements arecharged with a constant current, and further parasitic capacitance ischarged.

It is preferable that the anode potential be inverted before the cathodepotential is inverted. After a predetermined reverse bias voltageapplication period passes, the anode potential is returned and then thecathode potential is returned. In synchronism with the inversion of theanode potential, the potential of the monitor line 113 becomes High.

In the reverse bias voltage application period, the driving transistor12 and the monitor controlling transistor 111 are required to be on.

As a result of applying the reverse bias voltage to the light emittingelement, defects of the light emitting element 13 and the monitoringlight emitting element 66 can be improved to increase reliability.Further, the light emitting element 13 and the monitoring light emittingelement 66 may have an initial defect where an anode and a cathode areshort-circuited due to adhesion of foreign materials, a pinhole causedby a small projection in the anode or the cathode, and nonuniformity ofan electroluminescent layer. In the case where such an initial defectoccurs, lighting and non-lighting corresponding to a signal are notperformed and most current flows to a short-circuited element. As aresult, a problem that an image is not displayed well occurs. Thisdefect may occur in an arbitrary pixel.

In view of the aforementioned, in this embodiment mode, a reverse biasvoltage is applied to the light emitting element 13 and the monitoringlight emitting element 66, thereby a current partially flows to ashort-circuited portion, and the short-circuited portion generates heatto be oxidized or carbonized. As a result, the short-circuited portioncan be insulated, a current flows to the other parts of theshort-circuited portion, and the light emitting element 13 and themonitoring light emitting element 66 can operate normally. By applying areverse bias voltage in this manner, even in the case where an initialdefect occurs, the defect can be corrected. Note that such insulation ofthe short-circuited portion may be performed before shipment of adisplay device.

As time passes, another short-circuit between an anode and a cathode aswell as the initial defect may occur. Such a defect is also referred toas a progressive defect. According to the invention, a reverse biasvoltage is periodically applied to the light emitting element 13 and themonitoring light emitting element 66 so that, even when a progressivedefect occurs, the defect can be corrected and the light emittingelement 13 and the monitoring light emitting element 66 can operatenormally.

In addition, application of a reverse bias voltage can prevent imageburn-in. Image burn-in occurs by the light emitting element 13 which hasdegraded, though application of a reverse bias voltage can reduce thedegradation. As a result, image burn-in can be prevented.

In general, degradation of the light emitting element 13 and themonitoring light emitting element 66 progresses rapidly in the initialstage and gradually slows down as time passes. That is, in a pixel, thelight emitting element 13 and the monitoring light emitting element 66which have degraded in the initial stage do not degrade easily. As aresult, there occur variations in each light emitting element 13.Therefore, before shipment, when no image is displayed and the like, allthe light emitting elements 13 and the monitoring light emittingelements 66 emit light to cause degradation of elements which have notdegraded, so that the degradation level of all the elements can beaveraged. Such a configuration in which all the elements emit light maybe provided in a light emitting device.

Embodiment Mode 4

In this embodiment mode, another example of a pixel circuit isdescribed.

FIG. 2 shows a pixel circuit which can be used in the pixel portion ofthe invention. The pixel portion 40 includes a signal line Sx, ascanning line Gy, and a power source line Vx which are arranged inmatrix, and the pixel 10 is provided at each intersection thereof. Thepixel 10 has the switching transistor 11, the driving transistor 12, acapacitor 16, and the light emitting element 13.

A connection relation of the pixel is described. The switchingtransistor 11 is provided at the intersection of the signal line Sx andthe scanning line Gy. One electrode of the switching transistor 11 isconnected to the signal line Sx, while a gate electrode of the switchingtransistor 11 is connected to the scanning line Gy. One electrode of thedriving transistor 12 is connected to the power source line Vx, while agate electrode thereof is connected to the other electrode of theswitching transistor 11. The capacitor 16 is provided to hold agate-source voltage of the driving transistor 12. In this embodimentmode, one electrode of the capacitor 16 is connected to the power sourceline Vx, while the other electrode is connected to the gate electrode ofthe driving transistor 12. Note that the capacitor 16 is not required tobe provided in the case where a gate capacitance of the drivingtransistor 12 is large, a leakage current is small and the like. Thelight emitting element 13 is connected to the other electrode of thedriving transistor 12.

A driving method of such a pixel is described.

First, when the switching transistor 11 is turned on, a video signal isinputted from the signal line Sx. Charges are accumulated in thecapacitor 16 based on the video signal. When the charges accumulated inthe capacitor 16 exceed the gate-source voltage (Vgs) of the drivingtransistor 12, the driving transistor 12 is turned on. Then, a currentis supplied to the light emitting element 13 and the light emittingelement 13 emits light. At this time, the driving transistor 12 can beoperated in the linear region or the saturation region. When the drivingtransistor 12 is operated in the saturation region, it can supply aconstant current, while when operated in the linear region, it can beoperated at a low voltage to achieve lower power consumption.

A driving method of a pixel is hereinafter described with reference to atiming chart.

FIG. 8A shows a timing chart of one-frame period in the case wheresixty-frame images are rewritten per second. In the timing chart, theordinate shows a scanning line G (from the first row to the last row),while the abscissa shows time.

One-frame period has m (m is a natural number of two or more) subframeperiods SF1, SF2, . . . , SFm, which have writing periods Ta1, Ta2, . .. , Tam and display periods (lighting period) Ts1, Ts2, . . . , Tsm,respectively. One-frame period also has a reverse bias voltageapplication period. In this embodiment mode, the subframe periods SF1,SF2, and SF3, and the reverse bias voltage application period (FRB) areprovided in one-frame period as shown in FIG. 8A. During each subframeperiod, the writing periods Ta1 to Ta3 are sequentially performedfollowed by the display periods Ts1 to Ts3, respectively.

A timing chart illustrated in FIG. 8B shows a writing period, a displayperiod, and a reverse bias voltage application period of a certain row(i-th row). The reverse bias voltage application period appears afterthe writing period and the display period appear alternately. A periodwhich has the writing period and the display period is a forward biasvoltage application period.

A writing period Ta is divided into a plurality of operation periods. Inthis embodiment mode, the writing period is divided into two periods. Anerasing operation is performed during one of the two periods. While, awriting operation is performed during the other one of the two periods.To thus provide the erasing period and the writing period, a WE (WriteErase) signal is inputted. Other erasing operation, writing operation,and a signal are described in detail in the following embodiment mode.

Immediately before the reverse bias voltage application period, a periodin which switching transistors of all pixels are turned on at the sametime, that is, a period (on-period) in which all scanning lines areturned on is provided.

Immediately after the reverse bias voltage application period, a periodin which switching transistors of all pixels are turned off at the sametime, that is, a period (off-period) in which all scanning lines areturned off is provided.

Immediately before the reverse bias voltage application period, anerasing period (SE) is also provided. The operation in the erasingperiod can be performed in the same manner as the aforementioned erasingoperation. In the erasing period, data which has been written in thesubframe period immediately before the erasing period, in thisembodiment mode, SF3, is sequentially erased. This is because in theon-period, after the display period of pixels of the last row isfinished, the switching transistors are turned on all at once so thatpixels of the first row and the like have an unnecessary display period.

Such operations in the on-period, the off-period, and the erasing periodare performed by a driving circuit such as a scanning line drivingcircuit and a signal line driving circuit.

Note that the timing at which a reverse bias voltage is applied to thelight emitting element 13, in other words, the reverse bias voltageapplication period, is not limited to the one shown in FIGS. 8A and 8B.That is to say, the reverse bias voltage application period is notnecessarily provided in each frame period nor the latter half of oneframe period. Further, the on-period is only required to be providedimmediately before an applying period (RB), while the off-period is onlyrequired to be provided immediately after the applying period (RB). Theorder of reversing the anode potential and the cathode potential of thelight emitting element is not limited to the one shown in FIGS. 8A and8B. That is, the anode potential may be decreased after the cathodepotential is increased.

A layout example of the pixel circuit shown in FIG. 2 is illustrated inFIG. 3. A semiconductor film which constitutes the switching transistor11 and the driving transistor 12 is provided. Thereafter, a firstconductive film is provided with an insulating film which functions as agate insulating film interposed therebetween. The first conductive filmcan be used as gate electrodes of the switching transistor 11 and thedriving transistor 12, and a scanning line Gy. At this time, theswitching transistor 11 may have a double-gate structure.

Thereafter, a second conductive film is formed with an insulating filmwhich functions as an interlayer insulating film interposedtherebetween. The second conductive film can be used as a drain wiringand a source wiring of the switching transistor 11 and the drivingtransistor 12, and as the signal line Sx and the power source line Vx.At this time, the capacitor 16 can be formed by stacking the firstconductive film, the insulating film which functions as an interlayerinsulating film, and the second conductive film. The gate electrode ofthe driving transistor 12 and the other electrode of the switchingtransistor 11 are connected through a contact hole.

A pixel electrode 19 is formed in an opening which is provided in apixel. The pixel electrode 19 is connected to the other electrode of thedriving transistor 12. At this time, in the case where an insulatingfilm and the like is provided between the second conductive film and thepixel electrode, the pixel electrode is required to be connected to theother electrode of the driving transistor through a contact hole. In thecase where an insulating film and the like is not provided, the otherelectrode of the driving transistor 12 can be directly connected to thepixel electrode.

In the layout shown in FIG. 3, the first conductive film and the pixelelectrode may overlap each other to achieve a high aperture ratio. Insuch a region, a coupling capacitance may be generated, which isunwanted capacitance. The unwanted capacitance can be eliminated by thedriving method of the invention.

FIG. 4 shows a cross-sectional-view example cutting along lines A-B andB-C of FIG. 3.

A patterned semiconductor film is formed over an insulating substrate 20with a base film interposed therebetween. The insulating substrate 20can be formed using, for example, a glass substrate such as a bariumborosilicate glass and an aluminoborosilicate glass, a quartz substrate,a stainless (SUS) substrate and the like. Further, a substrate which isformed of synthetic resin having flexibility such as plastic typified byPET (polyethylene terephthalate), PEN (polyethylene naphthalate) and PBS(polyether sulfone) and acrylic generally tends to have a lower heatresistance temperature compared with other substrates. However, such asubstrate can be used if it can withstand a processing temperature inmanufacturing steps. The base film can be formed by using an insulatingfilm such as silicon oxide, silicon nitride, and silicon nitride oxide.

An amorphous semiconductor film is formed over the base film. The filmthickness of the amorphous semiconductor film is 25 to 100 nm(preferably 30 to 60 nm). Further, silicon germanium as well as siliconcan be used as the amorphous semiconductor film.

Next, the amorphous semiconductor film is crystallized if necessary toform a crystalline semiconductor film. Crystallization can be performedby using a furnace, laser irradiation, or light irradiation from a lamp(hereinafter referred to as lamp anneal), or a combination thereof. Forexample, the amorphous semiconductor film is doped with a metal element,and subjected to heat treatment by using the furnace, thereby forming acrystalline semiconductor film. Thus, it is preferable to dope the filmwith the metal element since the film can be crystallized at lowtemperature.

The crystalline semiconductor film formed in this manner is patternedinto a predetermined shape. The predetermined shape is a shape to be theswitching transistor 11 and the driving transistor 12 as shown in FIG.3.

Next, an insulating film is formed, which functions as a gate insulatingfilm. The insulating film is formed to cover the semiconductor film andhave a thickness of 10 to 150 nm, and preferably 20 to 40 nm. Forexample, a silicon oxynitride film, a silicon oxide film and the likecan be used and a monolayer structure or a stacked structure may beadopted.

A first conductive film which functions as a gate electrode is formedwith the gate insulating film interposed therebetween. The gateelectrode may have a monolayer structure or a stacked structure, thoughin this embodiment mode, a stacked structure of conductive films 22 aand 22 b is used. Each of the conductive films 22 a and 22 b may beformed of an element selected from Ta, W, Ti, Mo, Al, and Cu, or analloy material or a compound material which mainly contains theaforementioned elements. In this embodiment mode, a tantalum nitridefilm with a thickness of 10 to 50 nm, for example, 30 nm is formed asthe conductive film 22 a, and a tungsten film with a thickness of 200 to400 nm, for example, 370 nm is formed thereon as the conductive film 22b.

An impurity element is doped by using the gate electrode as a mask. Atthis time, in addition to a high-concentration impurity region, alow-concentration impurity region may be formed, which is referred to asan LDD (Lightly Doped Drain) structure. Specifically, a structure inwhich the low-concentration impurity region overlaps the gate electrodeis referred to as a GOLD (Gate-drain Overlapped LDD) structure.Specifically, an N-channel transistor may adopt a configurationincluding the low-concentration impurity region.

The low-concentration impurity region may cause unwanted capacitance.Therefore, in the case where a pixel is constituted by a TFT which hasthe LDD structure or the GOLD structure, it is preferable to use thedriving method of the invention.

Thereafter, insulating films 28 and 29 which function as an interlayerinsulating film 30 are provided. The insulating film 28 may be aninsulating film containing nitrogen, and in this embodiment mode, asilicon nitride film with a thickness of 100 nm is formed by a plasmaCVD method. The insulating film 29 can be formed by using an organicmaterial of an inorganic material. An organic material includespolyimide, acrylic, polyamide, polyimide amide, resist,benzocyclobutene, siloxane, or polysilazane. Note that siloxane is aresin which includes a Si—O—Si bond. Siloxane is composed of a skeletonformed by the bond of silicon (Si) and oxygen (O), in which an organicgroup containing at least hydrogen (such as an alkyl group or aromatichydrocarbon) is included as a substituent. Alternatively, a fluoro groupmay be used as the substituent. Further alternatively, a fluoro groupand an organic group containing at least hydrogen may be used as thesubstituent. Polysilazane is formed using as a starting material aliquid material containing a polymer material having the bond of silicon(Si) and nitrogen (N). An insulating film containing oxygen or nitrogensuch as silicon oxide (SiO_(x)), silicon nitride (SiN_(x)), siliconoxynitride (SiO_(x)N_(y)) (x>y), and silicon nitride oxide(SiN_(x)O_(y)) (x>y) (x, y=1, 2 . . . ) can be used as an inorganicmaterial. Further, the stacked structure of these insulating films maybe used as the insulating film 28. Specifically, in the case where asecond interlayer insulating film is formed using an organic material,planarity increases, while moisture and oxygen are absorbed into theorganic material. To prevent the absorption of moisture and oxygen, aninsulating film which has an inorganic material may be provided over theorganic material. An insulating film containing nitrogen is preferablyused as the inorganic material since alkali ions such as Na can beprevented from entering. An organic material is preferably used for theinsulating film 29 since planarity can be improved.

A contact hole is formed in the interlayer insulating film 30. Thus, asecond conductive film is formed, which functions as source and drainwirings 24 of the switching transistor 11 and the driving transistor 12,the signal line Sx, and the power source line Vx. The second conductivefilm can be formed using a film containing an element such as aluminum(Al), titanium (Ti), molybdenum (Mo), tungsten (W), or silicon (Si), oran alloy film containing these elements. In this embodiment mode, thesecond conductive film is formed by stacking a titanium (Ti) film, atitanium nitride (TiN) film, an aluminum-silicon (Al—Si) alloy film, anda titanium (Ti) film so as to have a thickness of 60 nm, 40 nm, 300 nm,and 100 nm respectively.

Thereafter, an insulating film 31 is provided to cover the secondconductive film. The insulating film 31 can be formed using any of thematerials of the interlayer insulating film 30 described above. A highaperture ratio can be achieved by providing such an insulating film 31.

A pixel electrode (also referred to as a first electrode) 19 is formedin the opening which is provided in the insulating film 31. The edge ofthe opening is preferably roundish so as to have a plurality ofcurvature radii in order to increase the step coverage of the pixelelectrode in the opening. The pixel electrode 19 may be formed using alight transmissive material such as ITO (Indium Tin Oxide), IZO (IndiumZinc Oxide) in which zinc oxide (ZnO) of 2 to 20% is mixed into indiumoxide, ITO-SiO_(x) (referred to as ITSO or NITO for convenience) inwhich silicon oxide (SiO₂) of 2 to 20% is mixed into indium oxide,organic indium, and organic tin. The pixel electrode 19 may also beformed using a non-light transmissive material such as tantalum,tungsten, titanium, molybdenum, aluminum, and copper as well as silver(Ag) or an alloy material or a compound material which mainly containsthe aforementioned elements. At this time, in the case where theinsulating film 31 is formed using an organic material to increaseplanarity, the surface planarity on which the pixel electrode is formedincreases, so that a constant voltage can be applied and a short-circuitcan be prevented.

Coupling capacitance may be generated in the region 430 in which thefirst conductive film overlaps the pixel electrode. The couplingcapacitance is unwanted capacitance. Such unwanted capacitance can beeliminated by the driving method of the invention.

Thereafter, an electroluminescent layer 33 is formed by a vapordeposition method or an inkjet method. The electroluminescent layer 33has an organic material or an inorganic material, and is constituted byarbitrarily combining an electron injection layer (EIL), an electrontransporting layer (ETL), a light emitting layer (EML), a holetransporting layer (HTL), a hole injection layer (HIL) and the like.Note that the boundaries between each layer are not necessarily clearlydefined, and there is also a case where materials of the respectivelayers are partially mixed with each other, which blurs the boundaries.Further, the structure of the electroluminescent layer is not limited tothe aforementioned stacked structure.

A second electrode 35 is formed by a sputtering method or a vapordeposition method. The first electrode (pixel electrode) 19 and thesecond electrode 35 of the electroluminescent layer (light emittingelement) function as an anode or a cathode depending on a pixelconfiguration.

The anode is preferably formed using a metal, an alloy, a conductivecompound, and a mixture thereof which have a high work function (workfunction of 4.0 eV or more). More specifically, it is possible to usegold (Au), platinum (Pt), Nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), and palladium(Pd), or nitride (TiN) of metal material and the like as well as ITO andIZO in which zinc oxide (ZnO) of 2 to 20% is mixed in indium oxide.

On the other hand, the cathode is preferably formed using a metal, analloy, a conductive compound, and a mixture thereof which have a lowwork function (work function of 3.8 eV or less). More specifically, itis possible to use an element which belongs to Group 1 or Group 2 of thePeriodic Table of the Elements, that is to say, an alkaline metal suchas Li and Cs, an alkaline-earth metal such as Mg, Ca, and Sr, an alloy(Mg: Ag, Al: Li) or a compound (LiF, CsF, CaF₂) containing these metals,and a transition metal which includes a rare-earth metal. Note thatsince the cathode is required to transmit light, these metals or alloyscontaining them are formed extremely thin and stacked with a metal(including an alloy) such as ITO.

Then, a protective film may be formed so as to cover the secondelectrode 35. As the protective film, a silicon nitride film or a DLCfilm can be used.

In this manner, the pixel of the light emitting device can be completed.

Embodiment Mode 5

Described in this embodiment mode is a configuration of the whole panelwhich has the pixel circuit shown in the aforementioned embodiment mode.

As shown in FIG. 12, the light emitting device of the invention has thepixel portion 40 in which the aforementioned plurality of pixels 10 arearranged in matrix, the first scanning line driving circuit 41, thesecond scanning line driving circuit 42, and the signal line drivingcircuit 43. The first scanning line driving circuit 41 and the secondscanning line driving circuit 42 may be arranged to face each otheracross the pixel portion 40, or arranged on any one of the four sides:left, right, top, and bottom of the pixel portion 40.

The signal line driving circuit 43 has a pulse output circuit 44, alatch 45, and a selection circuit 46. The latch 45 has a first latch 47and a second latch 48. The selection circuit 46 has a transistor 49(hereinafter referred to as a TFT 49) and an analog switch 50 asswitching means. The TFT 49 and the analog switch 50 are provided ineach column depending on a signal line. In addition, in this embodimentmode, an inverter 51 is provided in each column to generate an invertedsignal of a WE signal. Note that the inverter 51 is not necessarilyprovided in the case where the inverted signal of the WE signal issupplied externally.

A gate electrode of the TFT 49 is connected to a selection signal line52, and one electrode thereof is connected to a signal line while theother electrode is connected to a power source 53. The analog switch 50is provided between the second latch 48 and each signal line. In otherwords, an input terminal of the analog switch 50 is connected to thesecond latch 48, while an output terminal is connected to a signal line.The analog switch 50 has two control terminals, one of which isconnected to the selection signal line 52, while the other is connectedto the selection signal line 52 through the inverter 51. The powersource 53 has a potential which turns off the driving transistor 12 ineach pixel, and the potential of the power source 53 is at Low in thecase where the driving transistor 12 has N-channel conductivity, whilethe potential of the power source 53 is at High in the case where thedriving transistor 12 has P-channel conductivity.

The first scanning line driving circuit 41 has a pulse output circuit 54and a selection circuit 55. The second scanning line driving circuit 42has a pulse output circuit 56 and a selection circuit 57. Start pulses(G1SP, G2SP) are inputted to the pulse output circuits 54 and 56,respectively. Further, clock pulses (G1CK, G2CK) and inverted clockpulses (G1CKB, G2CKB) thereof are inputted to the pulse output circuits54 and 56, respectively.

The selection circuits 55 and 57 are connected to the selection signalline 52. Note that the selection circuit 57 included in the secondscanning line driving circuit 42 is connected to the selection signalline 52 through an inverter 58. That is to say, WE signals which areinputted to the selection circuits 55 and 57 through the selectionsignal line 52 are inverted from each other.

Each of the selection circuits 55 and 57 has a tri-state buffer. Thetri-state buffer is brought into an operation state in the case where asignal inputted from the selection signal line 52 is at H level, whilethe tri-state buffer is brought into a high impedance state in the casewhere the signal is at L level.

Each of the pulse output circuit 44 included in the signal line drivingcircuit 43, the pulse output circuit 54 included in the first scanningline driving circuit 41, and the pulse output circuit 56 included in thesecond scanning line driving circuit 42 has a shift register including aplurality of flip-flop circuits or a decoder circuit. When a decodercircuit is used as the pulse output circuits 44, 54, and 56, a signalline or a scanning line can be selected at random, which can preventpseudo-contour from occurring in the case where a time gray scale methodis adopted.

Note that the configuration of the signal line driving circuit 43 is notlimited to the aforementioned one, and a level shifter or a buffer maybe provided additionally. The configurations of the first scanning linedriving circuit 41 and the second scanning line driving circuit 42 arealso not limited to the aforementioned one, and a level shifter or abuffer may be provided additionally. Further, each of the signal linedriving circuit 43, the first scanning line driving circuit 41, and thesecond scanning line driving circuit 42 may have a protection circuit.

In the invention, a protection circuit may be provided. The protectioncircuit may include a plurality of resistor elements. For example, aP-channel transistor can be used as the plurality of resistor elements.The protection circuit can be provided in each of the signal linedriving circuit 43, the first scanning line driving circuit 41, and thesecond scanning line driving circuit 42. The protection circuit ispreferably provided between the pixel portion 40 and the signal linedriving circuit 43, the first scanning line driving circuit 41, or thesecond scanning line driving circuit 42. Such a protection circuitprevents degradation or destruction of elements due to staticelectricity.

In this embodiment mode, the light emitting device has a power sourcecontrol circuit 63. The power source control circuit 63 has a powersource circuit 61 which supplies power to the light emitting element 13and a controller 62. The power source circuit 61 has a first powersource 17 which is connected to a pixel electrode of the light emittingelement 13 through the driving transistor 12 and the power source lineVx. The power source circuit 61 also has a second power source 18 whichis connected to the light emitting element 13 through the power sourceline connected to an opposite electrode.

In such a power source circuit 61, when a forward bias voltage isapplied to the light emitting element 13 so that the light emittingelement 13 is supplied with a current and emits light, a potential ofthe first power source 17 is set to be higher than a potential of thesecond power source 18. On the other hand, when a reverse bias voltageis applied to the light emitting element 13, the potential of the firstpower source 17 is set to be lower than the potential of the secondpower source 18. Such a setting of the power source can be performed bysupplying a predetermined signal from the controller 62 to the powersource circuit 61.

In this embodiment mode, the light emitting device has the monitoringcircuit 64 and a control circuit 65. The control circuit 65 has theconstant current source and the buffer amplifier circuit. The monitoringcircuit 64 has the monitoring light emitting element 66, the monitorcontrolling transistor 111, and the inverter 112.

The control circuit 65 supplies to the power source control circuit 63 asignal which corrects a power source potential based on an output of themonitoring circuit 64. The power source control circuit 63 corrects apower source potential to be supplied to the pixel portion 40 based on asignal which is supplied from the control circuit 65.

In the light emitting device of the invention which has theaforementioned configuration, variation in a current value due to achange of ambient temperature and degradation with time can besuppressed, leading to improved reliability. Further, the monitorcontrolling transistor 111 and the inverter 112 prevents a current fromthe constant current source 105 from flowing to the monitoring lightemitting element 66 which is short-circuited, so that variations in acurrent value can be supplied to the light emitting element 13accurately.

Embodiment Mode 6

In this embodiment mode, an operation of the light emitting device ofthe invention which has the aforementioned configuration is describedwith reference to drawings.

First, an operation of the signal line driving circuit 43 is describedwith reference to FIG. 14A. A clock signal (hereinafter referred to asSCK), a clock inverted signal (hereinafter referred to as SCKB), and astart pulse (hereinafter referred to as SSP) are inputted to the pulseoutput circuit 44, and in accordance with the timing of these signals, asampling pulse is outputted to the first latch 47. The first latch 47 towhich data is inputted holds video signals from the first column to thelast column in accordance with the timing of the sampling pulse. Thevideo signals held in the first latch 47 are transferred to the secondlatch 48 at a time when a latch pulse is inputted.

Herein, an operation of the selection circuit 46 during each period isdescribed, on the assumption that a WE signal transmitted from theselection signal line 52 is at L level during a period T1 while at Hlevel during a period T2. Each of the periods T1 and T2 corresponds tohalf of a horizontal scanning period, and the period T1 refers to as afirst subgate selection period while the period T2 refers to as a secondsub gate selection period.

During the period T1 (the first subgate selection period), the WE signaltransmitted from the selection signal line 52 is at L level, thetransistor 49 is in an on-state, and the analog switch 50 is in anon-conductive state. Then, a plurality of signal lines S1 to Sn areelectrically connected to the power source 53 through the transistor 49which is arranged in each column. In other words, a plurality of signallines Sx have the same potential as the power source 53. At this time,the switching transistor 11 in the selected pixel 10 is turned on sothat the potential of the power source 53 is transmitted to the gateelectrode of the driving transistor 12 through the switching transistor11. Then, the driving transistor 12 is in an off-state so that nocurrent flows between both electrodes of the light emitting element 13and no light is emitted. Thus, independently of a state of a videosignal which is inputted to the signal line Sx, the potential of thepower source 53 is transmitted to the gate electrode of the drivingtransistor 12 so that the switching transistor 11 is brought into anoff-state, and light emission of the light emitting element 13 isforcibly stopped, which is an erasing operation.

During the period T2 (the second sub gate selection period), the WEsignal transmitted from the selection signal line 52 is at H level, thetransistor 49 is in an off-state, and the analog switch 50 is in aconductive state. Then, video signals of one row which are held in thesecond latch 48 are transmitted to each signal line Sx at a time. Atthis time, the switching transistor 11 in the pixel 10 is turned on, anda video signal is transmitted to the gate electrode of the drivingtransistor 12 through the switching transistor 11. In accordance withthe inputted video signal, the driving transistor 12 is turned on oroff, and the first electrode and the second electrode of the lightemitting element 13 have different potentials or the same potential.More specifically, when the driving transistor 12 is turned on, thefirst electrode and the second electrode of the light emitting element13 have different potentials so that a current flows to the lightemitting element 13, and light is emitted. Note that the current flowingto the light emitting element 13 is the same as the current flowingbetween the source and drain of the driving transistor 12.

On the other hand, when the driving transistor 12 is turned off, thefirst electrode and the second electrode of the light emitting element13 have the same potential, and no current flows to the light emittingelement 13. That is to say, the light emitting element 13 emits nolight. In this manner, in accordance with a video signal, the drivingtransistor 12 is brought into an on-state or an off-state, and the firstelectrode and the second electrode of the light emitting element 13 havedifferent potentials or the same potential, which is a writingoperation.

Next, the operation of the first scanning line driving circuit 41 andthe second scanning line driving circuit 42 is described. G1CK, G1CKB,and G1SP are inputted to the pulse output circuit 54, and in accordancewith the timing of these signals, pulses are outputted to the selectioncircuit 55 sequentially. Meanwhile, G2CK, G2CKB, and G2SP are inputtedto the pulse output circuit 56, in accordance with the timing of thesesignals, pulses are outputted to the selection circuit 57 sequentially.Potentials of the pulses which are supplied to the selection circuits 55and 57 of each of the i-th row, the j-th row, the k-th row, and the p-throw (i, j, k, and p are natural numbers, 1≦i, j, k, and p≦n) are shownin FIG. 14B.

Herein, described are operations of the selection circuit 55 included inthe first scanning line driving circuit 41 and the selection circuit 57included in the second scanning line driving circuit 42 during eachperiod, on the assumption that a WE signal transmitted from theselection signal line 52 is at L level during a period T1, while the WEsignal is at H level during a period T2 similarly to the description ofthe signal line driving circuit 43. Note that in a timing chart of FIG.14B, a potential of the gate line Gy (y is a natural number, 1≦y≦n) towhich a signal is transmitted from the first scanning line drivingcircuit 41 is described as VGy (41), while a potential of the gate lineto which a signal is transmitted from the second scanning line drivingcircuit 42 is described as VGy (42). VGy (41) and VGy (42) can besupplied by the same scanning line Gy.

During the period T1 (the first subgate selection period), the WE signaltransmitted from the selection signal line 52 is at L level. Then, an Llevel WE signal is inputted to the selection circuit 55 included in thefirst scanning line driving circuit 41, and the selection circuit 55 isbrought into a floating state. On the other hand, an inverted WE signal,namely an H level signal is inputted to the selection circuit 57included in the second scanning line driving circuit 42 so that theselection circuit 57 is brought into an operation state. That is to say,the selection circuit 57 transmits an H level signal (row selectionsignal) to a gate line Gi of the i-th row so that the gate line Gi hasthe same potential as that of the H level signal. In other words, thegate line Gi of the i-th row is selected by the second scanning linedriving circuit 42. As a result, the switching transistor 11 in thepixel 10 is in an on-state. A potential of the power source 53 includedin the signal line driving circuit 43 is transmitted to the gateelectrode of the driving transistor 12 so that the driving transistor 12is in an off-state and the potentials of the two electrodes of the lightemitting element 13 are made equal to each other. That is to say, duringthe period T1, the erasing operation in which the light emitting element13 emits no light is performed.

During the period T2 (the second subgate selection period), the WEsignal transmitted from the selection signal line 52 is at H level.Then, an H level WE signal is inputted to the selection circuit 55included in the first scanning line driving circuit 41 so that theselection circuit 55 is in an operation state. In other words, theselection circuit 55 transmits an H level signal to the gate line Gi ofthe i-th row so that the gate line Gi has the same potential as that ofthe H level signal. That is to say, the gate line Gi of the i-th row isselected by the first scanning line driving circuit 41. As a result, theswitching transistor 11 in the pixel 10 is in an on-state. A videosignal is transmitted from the second latch 48 included in the signalline driving circuit 43 to the gate electrode of the driving transistor12 so that the driving transistor 12 is in an on-state or an off-state,and the two electrodes of the light emitting element 13 have differentpotentials or the same potential. In other words, during the period T2,the writing operation in which the light emitting element 13 emits lightor no light is performed. On the other hand, an L level signal isinputted to the selection circuit 57 included in the second scanningline driving circuit 42, and the selection circuit 57 is brought into afloating state.

Thus, the gate line Gy is selected by the second scanning line drivingcircuit 42 during the period T1 (the first subgate selection period),while selected by the first scanning line driving circuit 41 during theperiod T2 (the second subgate selection period). That is to say, the,gate line is controlled by the first scanning line driving circuit 41and the second scanning line driving circuit 42 in a complementarymanner. During one of the first subgate selection period and the secondsubgate selection period, the erasing operation is performed, and thewriting operation is performed during the other.

Note that during the period in which the first scanning line drivingcircuit 41 selects the gate line Gi of the i-th row, the second scanningline driving circuit 42 does not operate (the selection circuit 57 is ina floating state), or transmits a row selection signal to gate lines ofrows other than the i-th row. Similarly, during the period in which thesecond scanning line driving circuit 42 transmits the row selectionsignal to the gate line Gi of the i-th row, the first scanning linedriving circuit 41 is in a floating state, or transmits the rowselection signal to gate lines of rows other than the i-th row.

According to the invention performing the aforementioned operation, thelight emitting element 13 can be forcibly turned off, which increasesthe duty ratio. Further, although the light emitting element 13 can beturned off forcibly, a TFT for discharging the charges of the capacitor16 is not required to be provided, thereby a high aperture ratio isachieved. With the high aperture ratio, luminance of the light emittingelement can be reduced with an increase in a light emitting area. Thatis to say, a driving voltage can be decreased to reduce powerconsumption.

Note that the invention is not limited to the aforementioned embodimentmode in which a gate selection period is divided into two. A gateselection period may be divided into three or more.

Embodiment Mode 7

Exemplified in this embodiment mode is a pixel configuration to whichthe driving method of the invention is applied. Note that the sameconfiguration as that shown in FIG. 2 is not described.

FIG. 9 shows a pixel configuration in which a third transistor 25 isprovided between two terminals of the capacitor 16 in addition to thepixel configuration shown in FIG. 2. The third transistor 25 has afunction of discharging charges accumulated in the capacitor 16 during apredetermined period. The third transistor 25 is also referred to as anerasing transistor. The predetermined period is controlled by an erasingscanning line Ry to which a gate electrode of the third transistor 25 isconnected.

For example, in the case where a plurality of subframe periods areprovided, the charges in the capacitor 16 are discharged by the thirdtransistor 25 during a short subframe period. As a result, a duty ratiocan be increased.

FIG. 10A shows a pixel configuration in which a fourth transistor 36 isprovided between the driving transistor 12 and the light emittingelement 13 in addition to the pixel configuration shown in FIG. 2. Asecond power source line Vax with a fixed potential is connected to agate electrode of the fourth transistor 36. Therefore, a current whichis supplied to the light emitting element 13 can be constantindependently of gate-source voltages of the driving transistor 12 andthe fourth transistor 36. The fourth transistor 36 is also referred toas a current controlling transistor.

FIG. 10B shows a pixel configuration in which the second power sourceline Vax with a fixed potential is provided in parallel to the scanningline Gy, which differs from the configuration shown in FIG. 10A.

Further, FIG. 10C shows a pixel configuration in which the gateelectrode of the fourth transistor 36 with a fixed potential isconnected to the gate electrode of the driving transistor 12, whichdiffers from the configurations shown in FIGS. 10A and 10B. In the pixelconfiguration which does not require a new power source line as shown inFIG. 10C, an aperture ratio can be maintained.

FIG. 11 shows a pixel configuration in which the erasing transistorshown in FIG. 9 is provided in addition to the pixel configuration shownin FIG. 10A. The charges in the capacitor 16 can be discharged by theerasing transistor. It is needless to say that the erasing transistorcan be provided in addition to the pixel configuration shown in FIG. 10Bor 10C.

That is to say, the invention is not limited to the aforementionedconfigurations and can be applied to any configuration.

Embodiment Mode 8

The invention can be applied to a light emitting device driven with aconstant current. Described in this embodiment mode is a case where achange with time is detected by the monitoring light emitting element,and a change with time of a light emitting element is compensated bycorrecting a video signal or a power source potential based on thedetection result.

In this embodiment mode, a first monitoring light emitting element and asecond monitoring light emitting element are provided. A constantcurrent is supplied from a first constant current source to the firstmonitoring light emitting element, while a constant current is suppliedfrom a second constant current source to the second monitoring lightemitting element. The current value supplied from the first constantcurrent source is made different from that supplied from the secondconstant current source so that the total amount of current flowing tothe first monitoring light emitting element is different from thatflowing to the second monitoring light emitting element. Then, thechanges with time of the first monitoring light emitting element and thesecond monitoring light emitting element progress at different rates.

The first monitoring light emitting element and the second monitoringlight emitting element are connected to an arithmetic circuit whichcalculates a potential difference between the first monitoring lightemitting element and the second monitoring light emitting element. Avoltage value calculated by the arithmetic circuit is supplied to avideo signal generating circuit. In the video signal generating circuit,based on the voltage value supplied from the arithmetic circuit, a videosignal supplied to each pixel is corrected. The change with time of thelight emitting element can be compensated according to theaforementioned configuration.

Note that a circuit such as a buffer amplifier circuit for preventingvariations in potential may be provided between each monitoring lightemitting element and each arithmetic circuit.

Note that in this embodiment mode, a pixel which has a configuration toperform a constant current drive may use, for example, a current mirrorcircuit and the like.

Embodiment Mode 9

The invention can be applied to a passive matrix light emitting device.The passive matrix light emitting device has a pixel portion which isformed over a substrate, a column signal line driving circuit which is,arranged at the periphery of the pixel portion, a row signal linedriving circuit, and a controller which controls the driving circuits.The pixel portion has column signal lines which are arranged in thecolumn direction, row signal lines which are arranged in the rowdirection, and a plurality of light emitting elements which are arrangedin matrix. The monitoring circuit 64 can be provided over the substrateon which the pixel portion is formed.

In the light emitting device of this embodiment mode, image data whichis inputted to the column signal line driving circuit, or a voltagewhich is generated from a constant voltage source can be corrected bythe monitoring circuit 64 in accordance with a temperature change and achange with time, and a light emitting device in which the effect ofboth the temperature change and the change with time is reduced can beprovided.

Embodiment Mode 10

An electronic device which is provided with a pixel portion including alight emitting element includes: a television set (simply referred to asa TV, or a television receiver), a digital camera, a digital videocamera, a mobile phone set (simply referred to as a cellular phone set,or a cellular phone), a portable information terminal such as PDA, aportable game machine, a monitor for a computer, a computer, a soundreproducing device such as a car audio set, an image reproducing deviceprovided with a recording medium such as a home game machine, and thelike. Specific examples thereof are described with reference to FIGS.15A to 15F.

A portable information terminal shown in FIG. 15A includes a main body9201, a display portion 9202 and the like. The light emitting device ofthe invention can be applied to the display portion 9202. That is tosay, according to the invention in which the power source potentialsupplied to the light emitting element is corrected by the monitoringlight emitting element, it is possible to provide a portable informationterminal in which the effect of variations in a current value of thelight emitting element due to a change of ambient temperature and achange with time is reduced.

A digital video camera shown in FIG. 15B includes a display portion9701, a display portion 9702 and the like. The light emitting device ofthe invention can be applied to the display portion 9701. According tothe invention in which the power source potential supplied to the lightemitting element is corrected by the monitoring light emitting element,it is possible to provide a digital video camera in which the effect ofvariations in a current value of the light emitting element due to achange of ambient temperature and a change with time is reduced.

A cellular phone shown in FIG. 15C includes a main body 9101, a displayportion 9102 and the like. The light emitting device of the inventioncan be applied to the display portion 9102. According to the inventionin which the power source potential supplied to the light emittingelement is corrected by the monitoring light emitting element, it ispossible to provide a cellular phone in which the effect of variationsin a current value of the light emitting element due to a change ofambient temperature and a change with time is reduced.

A portable television set shown in FIG. 15D includes a main body 9301, adisplay portion 9302 and the like. The light emitting device of theinvention can be applied to the display portion 9302. According to theinvention in which the power source potential supplied to the lightemitting element is corrected by the monitoring light emitting element,it is possible to provide a portable television set in which the effectof variations in a current value of the light emitting element due to achange of ambient temperature and a change with time is reduced. Thelight emitting device of the invention can be applied to various typesof television sets such as a small-sized television incorporated in aportable terminal such as a cellular phone, a medium-sized televisionwhich is portable, and a large-sized television (for example, 40 inchesin size or more).

A portable computer shown in FIG. 15E includes a main body 9401, adisplay portion 9402 and the like. The light emitting device of theinvention can be applied to the display portion 9402. According to theinvention in which the power source potential supplied to the lightemitting element is corrected by the monitoring light emitting element,it is possible to provide a portable computer in which the effect ofvariations in a current value of the light emitting element due to achange of ambient temperature and a change with time is reduced.

A television set shown in FIG. 15F includes a main body 9501, a displayportion 9502 and the like. The light emitting device of the inventioncan be applied to the display portion 9502. According to the inventionin which the power source potential supplied to the light emittingelement is corrected by the monitoring light emitting element, it ispossible to provide a television set in which the effect of variationsin a current value of the light emitting element due to a change ofambient temperature and a change with time is reduced.

Embodiment Mode 11

Described in this embodiment mode is a configuration of a panel whichcan perform a full-color display, and has a monitoring light emittingelement for each light emitting element which emits a different colorlight.

FIG. 16 shows a light emitting device in which the pixel portion 40, thesignal line driving circuit 43, the first scanning line driving circuit41, the second scanning line driving circuit 42, and monitoring circuits64R, 64G and 64B are provided over the insulating substrate 20. A lightemitting element using a light emitting material which emits a differentcolor light is provided in each pixel 10R, 10G, and 10B to perform afull-color display in the pixel 40. Each light emitting element isconnected to respective power sources 18R, 18G, and 18B. Note that lightemitting elements which emit the same color light are arranged in astripe shape.

Buffer amplifier circuits 110R, 110G, and 110B are provided between themonitoring circuits 64R, 64G and 64B, and the pixels 10R, 10G and 10B,respectively. The operation of the buffer amplifier circuits can beperformed by reference to Embodiment Mode 1.

The configuration of monitoring circuits 64R, 64G and 64B can beachieved by reference to Embodiment Mode 1. Specifically, each of themonitoring circuit, has a monitoring light emitting element including alight emitting material which emits each color light, a monitorcontrolling transistor connected to the monitoring light emittingelement, and an inverter of which an output terminal is connected to agate electrode of the monitor controlling transistor and an inputterminal is connected to one electrode of the monitor controllingtransistor and the monitoring light emitting element. Further, eachmonitoring light emitting element is connected to respective powersources 18MR, 18MG, and 18MB. Each monitor controlling transistor isconnected to respective constant current sources 105R, 105G, and 105Bthrough a monitor line. The monitor controlling transistor has afunction of controlling current supply from the monitor line to each ofthe plurality of monitoring light emitting elements. The monitor line isconnected to electrodes of the plurality of monitoring light emittingelements so that changes in the potentials of the electrodes can bemonitored. Further, the constant current source has a function to supplya constant current to the monitor line.

In the light emitting device having such a configuration, even in thecase where the light emitting elements which emit different color lightsdegrade at different rates, the degradation can be compensated by eachmonitoring light emitting element. That is to say, even when degradationprogresses at different rates for each light emitting element material,the degradation can be compensated by providing the monitoring lightemitting element for each light emitting element as shown in thisembodiment mode. As a result, a light emitting device in whichvariations in luminance of each color light emitting element due to achange of ambient temperature and a change with time are reduced can beprovided. Further, each of the monitoring circuits 64R, 64G and 64B ofthe invention preferably has the plurality of monitoring light emittingelements since the aforementioned variations in luminance can becorrected using the average value of these monitoring light emittingelements. Even if any one of the monitoring light emitting elements doesnot function due to a defect and the like, it can be replaced by theother monitoring light emitting elements.

Note that this embodiment mode describes a configuration in which lightemitting elements which emit the same color light are arranged in astripe shape, though the invention is not limited thereto. For example,in a pixel arranged in a delta shape, a monitoring light emittingelement may be provided for a light emitting element which emits adifferent color light.

Further, in this embodiment mode, the monitoring circuit 64B for a bluelight emitting element is provided on the left side of the pixel portion40, while the monitoring circuit 64R for a red light emitting elementand the monitoring circuit 64G for a green light emitting element areprovided on the right side of the pixel portion 40, though the inventionis not limited thereto. For example, monitoring circuits for all thelight emitting elements may be provided on the left side of the pixelportion, or any one of the monitoring circuits may be provided on theupside or the downside of the pixel portion. Note that in view of thewhole light emitting device, it is preferable that a region including amonitoring circuit is evenly dispersed.

1. A light emitting device comprising: a first transistor having a firstgate, a first source, and a first drain; a second transistor having asecond gate, a second source, and a second drain; a third transistorhaving a third gate, a third source, and a third drain; and a lightemitting element having a first electrode, a second electrode, and alight emitting layer between the first electrode and the secondelectrode, wherein the first gate is electrically connected to thesecond gate, one of the first electrode and the second electrode, andone of the third source and the third drain, and wherein one of thefirst source and the first drain is electrically connected to the thirdgate, and one of the second source and the second drain.
 2. A lightemitting device comprising: a first transistor having a first gate, afirst source, and a first drain; a second transistor having a secondgate, a second source, and a second drain; a third transistor having athird gate, a third source, and a third drain; a fourth transistorhaving a fourth gate, a fourth source, and a fourth drain; and a lightemitting element having a first electrode, a second electrode, and alight emitting layer between the first electrode and the secondelectrode, wherein the first gate is electrically connected to thesecond gate, one of the first electrode and the second electrode, andone of the third source and the third drain, wherein one of the firstsource and the first drain is electrically connected to the third gate,and one of the second source and the second drain, and wherein the otherone of the second source and the second drain is electrically connectedto one of the fourth source and the fourth drain.
 3. A light emittingdevice comprising: a first transistor having a first gate, a firstsource, and a first drain; a second transistor having a second gate, asecond source, and a second drain; a third transistor having a thirdgate, a third source, and a third drain; a fourth transistor having afourth gate, a fourth source, and a fourth drain; a fifth transistorhaving a fifth gate, a fifth source, and a fifth drain; a light emittingelement having a first electrode, a second electrode, and a lightemitting layer between the first electrode and the second electrode,wherein the first gate is electrically connected to the second gate, oneof the first electrode and the second electrode, and one of the thirdsource and the third drain, wherein one of the first source and thefirst drain is electrically connected to the third gate, and one of thesecond source and the second drain, wherein the other one of the secondsource and the second drain is electrically connected to one of thefourth source and the fourth drain, and wherein the fourth gate iselectrically connected to the fifth gate.
 4. A light emitting devicecomprising: a first transistor having a first gate, a first source, anda first drain; a second transistor having a second gate, a secondsource, and a second drain; a third transistor having a third gate, athird source, and a third drain; and a first light emitting elementhaving a first electrode, a second electrode, and a first light emittinglayer between the first electrode and the second electrode; a fourthtransistor having a fourth gate, a fourth source, and a fourth drain; asecond light emitting element having a third electrode, a fourthelectrode, and a second light emitting layer between the third electrodeand the fourth electrode; and a buffer amplifier circuit, wherein thefirst gate is electrically connected to the second gate, one of thefirst electrode and the second electrode, and one of the third sourceand the third drain, wherein one of the first source and the first drainis electrically connected to the third gate, and one of the secondsource and the second drain, wherein the other one of the third sourceand the third drain and one of the fourth source and the fourth drainare electrically connected to the buffer amplifier circuit, and whereinthe other one of the fourth source and the fourth drain is electricallyconnected to one of third electrode and the fourth electrode.
 5. A lightemitting device according to claim 1, further comprising a constantcurrent source electrically connected to the other one of the thirdsource and the third drain.
 6. A light emitting device according toclaim 2, further comprising a constant current source electricallyconnected to the other one of the third source and the third drain.
 7. Alight emitting device according to claim 3, further comprising aconstant current source electrically connected to the other one of thethird source and the third drain.
 8. A light emitting device accordingto claim 4, further comprising a constant current source electricallyconnected to the other one of the third source and the third drain.
 9. Alight emitting device according to claim 1, further comprising a bufferamplifier circuit electrically connected to the other one of the thirdsource and the third drain.
 10. A light emitting device according toclaim 2, further comprising a buffer amplifier circuit electricallyconnected to the other one of the third source and the third drain. 11.A light emitting device according to claim 3, further comprising abuffer amplifier circuit electrically connected to the other one of thethird source and the third drain.
 12. A light emitting device accordingto claim 1, wherein each of the first transistor and the thirdtransistor is a p-channel transistor, and the second transistor is ann-channel transistor.
 13. A light emitting device according to claim 2,wherein each of the first transistor and the third transistor is ap-channel transistor, and each of the second transistor and the fourthtransistor is an n-channel transistor.
 14. A light emitting deviceaccording to claim 3, wherein each of the first transistor, the thirdtransistor and the fourth transistor is a p-channel transistor, and eachof the second transistor and the fifth transistor is an n-channeltransistor.
 15. A light emitting device according to claim 4, whereineach of the first transistor, the third transistor and the fourthtransistor is a p-channel transistor, and the second transistor is ann-channel transistor.
 16. A light emitting device according to claim 1,wherein the light emitting device is applied to an electronic deviceselected from the group consisting of a television receiver, a portableinformation terminal, a digital video camera, a cellular phone, and aportable computer.
 17. A light emitting device according to claim 2,wherein the light emitting device is applied to an electronic deviceselected from the group consisting of a television receiver, a portableinformation terminal, a digital video camera, a cellular phone, and aportable computer.
 18. A light emitting device according to claim 3,wherein the light emitting device is applied to an electronic deviceselected from the group consisting of a television receiver, a portableinformation terminal, a digital video camera, a cellular phone, and aportable computer.
 19. A light emitting device according to claim 4,wherein the light emitting device is applied to an electronic deviceselected from the group consisting of a television receiver, a portableinformation terminal, a digital video camera, a cellular phone, and aportable computer.